1. Field of the Invention
This invention relates to what is called a thin magnetic metal film type magnetic recording medium, in which a thin magnetic metal film is formed as a magnetic layer on a non-magnetic base by physical vapor deposition such as vacuum evaporation or sputtering.
2. Prior Art
The magnetic recording medium so far widely used in the prior art is of the coated type in which ferromagnetic oxide powders such as powders of .gamma.-Fe.sub.2 O.sub.3, Co-containing .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-containing Fe.sub.3 O.sub.4, berthollide compounds of .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, Co-containing berthollide compounds or powdered magnetic materials consisting essentially of Fe, Co or Ni are dispersed in an organic binder such as vinyl chloride-vinyl acetate copolymer, polyester resin or polyurethane resin to produce a magnetic paint which is then coated on the non-magnetic base and dried to produce the coated tape.
With increase in the demand for a high-density magnetic recording, what is called the thin magnetic metal film type magnetic recording medium obtained by directly coating a ferromagnetic metal material such as Co-Ni alloy on a non-magnetic base of a polyester or polyimide film by plating or physical vapor deposition such as vacuum evaporation, sputtering or ion plating, has been proposed and attracted general attention. This thin magnetic metal film type magnetic recording medium has a number of advantages such as superior coercive force, squareness ratio and electro-magnetic conversion characteristics in the short wavelength range, extremely small losses in thickness during reproduction and extremely small demagnetization during recording because of the reduction in thickness of the magnetic layer and the improved packing density of the magnetic material because it is no longer necessary to admix a non-magnetic organic binder into the magnetic layer.
However, in the aforementioned thin magnetic metal film type magnetic recording medium, durability or running properties are occasionally lowered due to increased effective contact area due in turn to the high surface smoothness of the magnetic layer resulting in increased frictional coefficients and sticking phenomenon and hence a demand is raised for improving these properties. In general, the magnetic recording medium is placed under a condition of high speed relative movement with the magnetic head in the course of recording and reproduction of magnetic signals. It is required that the tape running will occur smoothly and under stable conditions, while the wear or damage due to contact with the magnetic head should be minimized.
It has so far been tried to improve the durability and the running properties by forming a protective film by coating a lubricant on the magnetic layer, that is, on the surface of the thin magnetic metal film.
When the protective film is formed by coating the lubricant in the above described manner, it is required that the protective film shows good adhesivity to the thin magnetic metal film while also showing a high lubricating effect. The adhesivity and lubricating properties should remain excellent both under high temperature high humidity conditions as met in tropical and semi-tropical regions and under low temperature conditions as met in frigid regions.
However, the lubricant so far used has a limited operating temperature range and tends to solidify or become frozen under a low temperature such as 0.degree. to -5.degree. C. so that its lubricating properties are not exhibited satisfactorily.